(1) Field of the Invention
The present invention relates to a system and method for controlling a fuel supply to an internal combustion engine having a fuel supply cut-off function which exhibits during a predetermined engine deceleration condition.
(2) Background of the Art
It is generally necessary to supply an appropriate amount of fuel to the engine according to an outstanding engine operating condition when a supply of fuel to the engine is resumed (a fuel supply cut-off is released, so called, recovery of fuel supply) upon completion of the fuel supply cut-off in order to maintain a driveability of a vehicle in which the engine is mounted.
A Japanese Patent Application First Publication (Tokkai) sho 55-125335 published on Sept. 27, 1980 exemplifies a previously proposed fuel supply controlling system applied to the engine.
In the above-identified Japanese Patent Application Publication, the fuel supply controlling system detects an intake air quantity per revolution of the engine as an engine load, calculates a basic fuel injection quantity according to the intake air quantity, and injects fuel whose quantity corresponds to the basic fuel injection quantity at a predetermined fuel injection timing synchronized with the engine revolution into a vicinity of an intake port of the engine.
In addition, the previously proposed system cuts the fuel supply off when the engine revolution speed is reduced from a high speed range to a low speed range (fuel supply cut-off) so as to drive the engine with a negative torque due to a running inertia force of a vehicle in which the engine and system are mounted, thus preventing the generation of uncombusted fuel and increasing a fuel saving.
When the engine revolution speed is reduced to a predetermined fuel recovery speed (an engine revolution speed at which the fuel supply cut-off is released and the resumption of fuel injection is carried out), the fuel injection is immediately restarted irrespective of the predetermined injection timing so as to generate a positive engine torque. In this case, to avoid a torque variation at the time of fuel recovery due to a difference between the output of an intake air quantity sensor and air quantity actually supplied into each combustion chamber of the engine, the quantity of fuel supply at the time of fuel recovery (recovery fuel quantity) is appropriately corrected with a predetermined correction coefficient.
Furthermore, at the initial stage of fuel recovery, when the fuel cut-off interval exceeds a predetermined value of time, a constant quantity (a fuel recovery increment quantity) of fuel other than the above-described fuel quantity irrespective of the fuel cut-off is supplied to the engine (normally corresponds to a quantity of injection by one pulse) so that the vehicle driveability is maintained.
However, there is a problem in the above-described previously proposed fuel supply controlling system.
That is to say, since the fuel recovery increment quantity at the time of fuel recovery is constant irrespective of the engine operating condition and the system determines the presence or absence of the fuel increment quantity at the time of fuel recovery on the basis of only the interval of time during which the fuel supply cut-off is carried out, the vehicle driveability becomes worsened due to an influence of the non-negligible presence of a residual quantity of fuel remaining on a wall of the intake air passage of the engine and not always providing a most appropriate state of combustion at the time of fuel recovery.
For example, air-fuel mixture ratio in the engine becomes excessively rich when the above-described residual quantity of fuel increases. On the other hand, as the above-described residual quantity of fuel is reduced, the air-fuel mixture ratio becomes lean when fuel recovery is started. Therefore, the effect of improvement in an acceleration response characteristic of the vehicle is insufficient in the case of the above-described fuel supply controlling system.
In addition, the above-described problem causes reductions of exhaust emission effect and fuel economy.
In this way, a better improvement in the acceleration response characteristic needs to be achieved since in the previously proposed system a condition of the engine before the fuel supply cut-off is carried out (for example, the residual quantity of fuel remaining onto the intake air passage) is not taken into consideration upon determination of fuel recovery increment quantity.